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Learning Objectives   Understand process of somitogenesis  segmental pattern of somitomeres and somites along the neural tube.   Review the adult derivatives of each of the three morphological subdivisions of the somites  dermatome, sclerotome and myotome.   Understanding of patterns of osteogenesis from the sclerotome.   Understanding the process of myogenesis.   Examples of congenital malformations in development of the skeleton.

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Somitogenesis(1)   Process of segmentation  development of axial system  vertebrae, muscles and innervation   Somites form from paraxial mesoderm in anterior-posterior gradient, begins at neurulation.   Two parallel columns of mesodermal cells form along the longitudinal axis, on each side of the notochord and neural tube.   Transverse fissures form in the columns  forming somitomeres in cranial- caudal direction.   First seven pairs of cranial somitomeres form head mesenchyme  migrate, form masticatory and facial muscles.   Somite 8-46(rabbit) become segmented into block-like somites. Number of pairs constant for each species.   Mechanisms of compaction  laminin, collagen and fibronectin increases cell-to- cell adhesions and gap junctions.

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Summary   Somitogenesis is the process of segmentation of the paraxial mesoderm in anterior-posterior gradient, begins at neurulation.   The first seven pairs of cranial somitomeres form head mesenchyme,migrate to specific regions and form masticatory and facial muscles.   Somite 8-46(rabbit) become segmented into block-like somites. Number of pairs constant for each species.   Each somite has three morphological regions.Ventromedial  sclerotome  chondrocytes  form axial skeleton and syndetome from within sclerotome form tendons. The dorsolateral layer  dermatome, form dermis of skin.Ventral layer  myotome; form axial and appendicular muscles.   The molecular mechanisms regulating osteogenesis and myogenesis are transcription factors, growth factors,the Hox gene, Shh,BMP, and Wnt genes.   Few examples of common congenital malformations in development of the axial skeleton and in limb development